docs: def set_index(self, field, value):
          """
          set_index(field, value)
  
          Works like :meth:`add_index`, but ensures that there is only
          one index on given field. If other found, then removes it
          first.
  
          :param field: The index field.
          :type field: string
          :param value: The index value.
          :type value: string or integer
          :rtype: :class:`RiakObject <riak.riak_object.RiakObject>`
          """
          to_rem = set((x for x in self.indexes if x[0] == field))
          self.indexes.difference_update(to_rem)
          return self.add_index(field, value)